268 research outputs found
The Oxygen Abundance of Nearby Galaxies from Sloan Digital Sky Survey Spectra
We have derived the oxygen abundance for a sample of nearby galaxies in the
Data Release 5 of the Sloan Digital Sky Survey (SDSS) which possess at least
two independent spectra of one or several HII regions with a detected
[OIII]4363 auroral line. Since, for nearby galaxies, the [OII]3727 nebular line
is out of the observed wavelength range, we propose a method to derive (O/H)_ff
abundances using the classic Te method coupled with the ff relation. (O/H)_7325
abundances have also been determined, based on the [OII]7320,7330 line
intensities, and using a small modification of the standard Te method. The
(O/H)_ff and (O/H)_7325 abundances have been derived with both the one- and
two-dimensional t_2 - t_3 relations. It was found that the (O/H)_ff abundances
derived with the parametric two-dimensional t_2 - t_3 relation are most
reliable. Oxygen abundances have been determined in 29 nearby galaxies, based
on 84 individual abundance determinations in HII regions. Because of our
selection methods, the metallicity of our galaxies lies in the narrow range 8.2
< 12 + log (O/H) < 8.4. The radial distribution of oxygen abundances in the
disk of the spiral galaxy NGC 4490 is determined for the first time.Comment: 39 pages, 10 figures, 4 tables, accepted for publication in the
Astrophysical Journa
Self-similar signature of the active solar corona within the inertial range of solar-wind turbulence
We quantify the scaling of magnetic energy density in the inertial range of solar-wind turbulence seen
in situ at 1 AU with respect to solar activity. At solar maximum, when the coronal magnetic field is
dynamic and topologically complex, we find self-similar scaling in the solar wind, whereas at solar
minimum, when the coronal fields are more ordered, we find multifractality. This quantifies the solar-wind
signature that is of direct coronal origin and distinguishes it from that of local MHD turbulence, with
quantitative implications for coronal heating of the solar wind
A dynamical model of surrogate reactions
A new dynamical model is developed to describe the whole process of surrogate
reactions; transfer of several nucleons at an initial stage, thermal
equilibration of residues leading to washing out of shell effects and decay of
populated compound nuclei are treated in a unified framework. Multi-dimensional
Langevin equations are employed to describe time-evolution of collective
coordinates with a time-dependent potential energy surface corresponding to
different stages of surrogate reactions. The new model is capable of
calculating spin distributions of the compound nuclei, one of the most
important quantity in the surrogate technique. Furthermore, various observables
of surrogate reactions can be calculated, e.g., energy and angular distribution
of ejectile, and mass distributions of fission fragments. These features are
important to assess validity of the proposed model itself, to understand
mechanisms of the surrogate reactions and to determine unknown parameters of
the model. It is found that spin distributions of compound nuclei produced in
O+U O+U and O+U
O+U reactions are equivalent and much less than
10, therefore satisfy conditions proposed by Chiba and Iwamoto (PRC 81,
044604(2010)) if they are used as a pair in the surrogate ratio method.Comment: 17 pages, 5 figure
A nonextensive entropy approach to solar wind intermittency
The probability distributions (PDFs) of the differences of any physical
variable in the intermittent, turbulent interplanetary medium are scale
dependent. Strong non-Gaussianity of solar wind fluctuations applies for short
time-lag spacecraft observations, corresponding to small-scale spatial
separations, whereas for large scales the differences turn into a Gaussian
normal distribution. These characteristics were hitherto described in the
context of the log-normal, the Castaing distribution or the shell model. On the
other hand, a possible explanation for nonlocality in turbulence is offered
within the context of nonextensive entropy generalization by a recently
introduced bi-kappa distribution, generating through a convolution of a
negative-kappa core and positive-kappa halo pronounced non-Gaussian structures.
The PDFs of solar wind scalar field differences are computed from WIND and ACE
data for different time lags and compared with the characteristics of the
theoretical bi-kappa functional, well representing the overall scale dependence
of the spatial solar wind intermittency. The observed PDF characteristics for
increased spatial scales are manifest in the theoretical distribution
functional by enhancing the only tuning parameter , measuring the
degree of nonextensivity where the large-scale Gaussian is approached for
. The nonextensive approach assures for experimental studies
of solar wind intermittency independence from influence of a priori model
assumptions. It is argued that the intermittency of the turbulent fluctuations
should be related physically to the nonextensive character of the
interplanetary medium counting for nonlocal interactions via the entropy
generalization.Comment: 17 pages, 7 figures, accepted for publication in Astrophys.
Detection of Cold Atomic Clouds in the Magellanic Bridge
We report a detection of cold atomic hydrogen in the Magellanic Bridge using
21-cm absorption spectroscopy toward the radio source B0312-770. With a column
density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10
and a maximum 21-cm emission brightness temperature of 1.4 K, this line of
sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm
absorption and emission spectroscopy toward 7 other low column density
sightlines on the periphery of the LMC and SMC reveal absorption toward one
additional background radio source behind the SMC with tau=0.03. The data have
typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and
sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold
atomic phase which is probably accompanied by molecular condensations in the
tenuous interstellar medium of the Bridge region. Young OB stars observed in
the Magellanic Bridge could form "in situ" from these cold condensations rather
than migrate from regions of active star formation in the main body of the SMC.
The existence of cold condensations and star formation in the Magellanic Bridge
might be understood as a small scale version of the mechanism that produces
star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the
Astronomical Journa
The UV Upturn in Elliptical Galaxies as an Age Indicator
We show that the UV flux of old stellar systems can tell us about their ages.
Two independent populations synthesis groups that have had wildly different
views have here worked together and generated two solutions that can be easily
tested using space telescopes. Proposed tests will constrain the ages of giant
Es, that are often considered the oldest populations in the universe, and thus
cosmology.Comment: LaTeX and 11 eps figures Accepted for publication in Ap
The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
(Abridged) We use high S/N spectra of 20 HII regions in the giant spiral
galaxy M101 to derive electron temperatures for the HII regions and robust
metal abundances over radii R = 0.19-1.25 Ro (6-41 kpc). We compare the
consistency of electron temperatures measured from the [O III]4363, [N II]5755,
[S III]6312, and [O II]7325 auroral lines. Temperatures from [O III], [S III],
and [N II] are correlated with relative offsets that are consistent with
expectations from nebular photoionization models. However, the temperatures
derived from the [O II]7325 line show a large scatter and are nearly
uncorrelated with temperatures derived from other ions. Our derived oxygen
abundances O/H are well fitted by an exponential distribution over six disk
scale lengths, from approximately 1.3 solar in the center to 1/15 solar in the
outermost region studied (for solar 12 + log (O/H)=8.7). We measure significant
radial gradients in N/O and He/H abundance ratios, but relatively constant S/O
and Ar/O. Our abundances are systematically lower by 0.2-0.5 dex than those
derived from the most widely used strong-line "empirical" abundance indicators.
We suspect that most of the disagreement with the strong-line abundances arises
from uncertainties in the nebular models that are used to calibrate the
"empirical" scale, and that strong-line abundances derived for HII regions and
emission-line galaxies are as much as a factor of two higher than the actual
oxygen abundances. However other explanations, such as the effects of
temperature fluctuations on the auroral line based abundances cannot be
completely ruled out. These results point to the need for direct abundance
determinations of a larger sample of extragalactic HII regions, especially for
objects more metal-rich than solar.Comment: 50 pages, 14 figures, 8 tables. Accepted by Ap
On the Progenitor of the Type II Supernova 2004et in NGC 6946
Supernova (SN) 2004et is the eighth historical SN in the nearby spiral galaxy
NGC 6946. Here we report on early photometric and spectroscopic monitoring of
this object. SN 2004et is a Type II event, exhibiting a plateau in its light
curves, but its spectral and color evolution appear to differ significantly
from those of other, more normal Type II-plateau (II-P) SNe. We have analyzed
Canada-France-Hawaii Telescope (CFHT) images of the host galaxy taken prior to
the SN explosion, identifying a candidate progenitor for the SN. The star's
absolute magnitude and intrinsic color imply that it was a yellow, rather than
red, supergiant star, with an estimated zero-age main sequence mass of
. Although this mass estimate is consistent with
estimates and upper limits for the progenitors of other, more normal SNe II-P,
the SN 2004et progenitor's unusual color could further imply a pre-explosion
evolutionary history analogous to, but less extreme than, that for the
progenitors of the peculiar Type II-P SN 1987A or the Type IIb SN 1993J. The
identity of the progenitor candidate needs to be verified when the SN has
significantly dimmed.Comment: To appear in PASP (Feb 2005). A high resolution PostScript version is
available at http://astron.berkeley.edu/~weidong/ms_04et.ps.g
Calcium II Triplet Spectroscopy of LMC Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Abridged Abstract -
Utilizing the FORS2 instrument on the VLT, we have obtained near infrared
spectra for more than 200 stars in 28 populous LMC clusters. This cluster
sample spans a large range of ages (~ 1-13 Gyr) and metallicities (-0.3 >
[Fe/H] > -2.0) and has good areal coverage of the LMC disk. The strong
absorption lines of the Calcium II triplet are used to derive cluster radial
velocities and abundances. We determine mean cluster velocities to typically
1.6 km/s and mean metallicities to 0.04 dex (random error). For eight of these
clusters, we report the first spectroscopically determined metallicities based
on individual cluster stars, and six of these eight have no published radial
velocity measurements.
(continued in paper)Comment: 26 pages of text plus 14 figures and 6 tables. Accepted for
publication in AJ. Scheduled for Vol. 132, No. 4 (October 2006
Molecular gas and dust in Arp 94: The formation of a recycled galaxy in an interacting system
We present new results for the molecular gas, dust emission and the ionized
gas in J1023+1952, an HI rich intergalactic star-forming tidal dwarf galaxy
candidate. It is located at the projected intersection of two faint stellar
tidal streams wrapped around the interacting pair of galaxies NGC 3227/6
(Arp~94). Using the IRAM 30m telescope, emission from CO(1-0) and CO(2-1) was
detected across the entire extent of the neutral hydrogen cloud associated with
J1023+1952, a region of the size of 8.9_2\simtwo lower (FWHM of 30 - 70 \kms) than elsewhere in the cloud (FWHM of 80
- 120 \kms). Thus the kinematics of the gas, in addition to its column density,
seems to be a crucial factor in triggering SF. Optical/infrared spectral energy
distributions (SEDs) and Halpha photometry confirm that all the knots are
young, with a tentative age sequence from the south-west (oldest knots) to the
north-east (youngest). Optical spectroscopy of the brightest SF region allowed
us to determine the metallicity (12+log(O/H) = 8.6 +- 0.2) and the extinction
(A_B$=2.4). This shows that J1023+1952 is made from metal-enriched gas which
is inconsistent with the hypothesis that it represents a pre-existing dwarf
galaxy. Instead, it must be formed from recycled, metal-enrichd gas, expelled
from NGC 3227 or NGC 3226 in a previous phase of the interaction.Comment: accepted for publication in ApJ, 20 pages, 11 figure
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